I. Field of the Invention
This invention relates to computer systems, and in particular, it relates to an optical bus structure to couple electronic devices. This invention is also directed to a method of manufacturing an optical bus that provides a system for optical connections between electronic cards or modules in a computer system.
II. Description of the Related Art
As computer systems become more complex, there is a continuing requirement for driving signals at faster clock rates while at the same time minimizing power, noise, and electro-magnetic interference (EMI). Within such systems, the computer bus forms the primary vehicle by which communication between electronic subsystems takes place. In its most basic form, the bus is traditionally a series of electrical lines interconnecting the modules in the computer. Those modules connect to the bus by means of tap lines. In this implementation, the primary bus lines will not be broken, or rooted through any of the computer modules. Communication between modules takes place through the bus.
Given current limitations in computer technology, the electrical transmission characteristics of the bus are defined by the material properties of the bus components physical geometry of the components (spacing) together with the clock speed of the signals. Those characteristics, in part generated by limitations in materials and in part by limitations in electronic components generated, determine the switching speed at which signals as well as the presence of noise within the bus conductor occurs.
The dual requirements of faster speed and lower power consumption have been considered mutually inconsistent. That is, attempts to drive signals at a faster rate carries with it an increase in power consumption together with an increase in system noise. To date, there has been no satisfactory solution for the mutually exclusive requirements of high speed data transmission, noise dissipation and reduction in power requirements.
For example, if the area of a transmission line decreases, a transmission line inductance will increase. High inductance in turn causes a large voltage drop across the line tending to degrade the signal and require more power to maintain signal level at an acceptable value. While this problem can be alleviated by the use of a transmission line of greater area, a shorter path or slower switching that solution tends to reduce overall transmission speed of data between modules.
Moreover, by electrically coupling boards to the bus, impedance matching is required. Generally, resistors are used to accomplish this matching, which in themselves, draw power thus increasing the overall power requirements of the system.
In order to avoid the problems of electrically coupling modules in a computer, attention has shifted to optical techniques of data transmission within a computer architecture. Representative is U.S. Pat. No. 4,732,466, which employs a circuit board having an optical data bus. In this system each printed circuit board includes an optical fiber network which in a composite sense forms an optical bus to interconnect computer components on the same printed circuit board. While broadly relating to the concept of optical data transmission between computer elements, the system of the '446 patent has a number of serious shortcomings. First, there is no provision in the system for minimizing optical reflections in the structure. Reflections will occur at various interfaces causing optical signals to propagate back along the waveguides. This is true whether the waveguides are glass, polymer or any other material. Such reflection seriously impacts the utility of an optical bus structure in computer applications. Reflections will most likely induce enough extraneous optical "noise" that the device would be essentially inoperative.
Another important shortcoming is available optical systems not covered by the '446 patent is the ground rule for data bus transmission to provide a uniform distribution of optical power among all opto-electronic devices. That is, in computer architecture it is important to distribute optical power from one emitter equally to various receivers. Such is required in computer bus applications where a device receiving the optical systems may be plugged into the optical bus at any point. It must receive proper optical signals at each position. This requirement in turn mandates a uniform distribution of optical power to each "tap" from the wave guide which comprises the optical bake plane.
Rather, in the '446 patent, the structure is such that the received optical signal will diminish at successive receiver according to R(1-R) to the exponent N where, R is the power reflected from each splitting device and N is the number of such devices. As such, each splitter in the '446 patent will receive approximately 15% of the optical energy with the tenth opticoelectronic module receiving four times less optical signal as the second module. Such a system is essentially useless in digital signal transmission. The '446 system can not meet a standing requirement and current computer architectures that any circuit card be pluggable into any slot in a bus. Consequently, an optical bus must also distribute optical power uniformly otherwise digital signal values cannot be maintained.
Reference is made to U.S. Pat. No. 4,063,083, which is also directed to data communication between computer systems by employing an optical data bus. While employing optical paths to interconnect computer circuit elements, the '083 patent provides only for point-to-point interconnections. This is accomplished by means of having optical communication established between two points by employing arrays of light emitting diodes and photoconductors. However, in the context of computer architecture a standing requirement is to provide interconnections with multiple sources and sinks per signal line. Moreover, given the point-to-point interconnection scheme of the '083 patent, electronic selection of specific optical paths by transmitter and receiver are required. Thus, active repeaters are used on the optical communication link. Specifically, the optical signal is sensed by a photosensitive device, converted into electrical form, conducted electrically to an adjacent photoemitter and reconverted into optical form. While an optical bus is broadly used, it carries with it the expense of additional components and signaled delay which is a consequence of multiple conversions.
Other techniques have been proposed in the art for using optical couplers for purposes of data transmission. U.S. Pat. No. 4,400,054 relates to an optical coupler used to transmit data from any one of N small optical channels by funneling to a single "large" optical channel. This technique does not provide an optical interconnect scheme for purposes of computer bus applications where all optical ports must be interconnected to allow by-directional communication between any two device. That is, the system does not provide for any direct optical intercommunication between the N "small optical channels".
A different type of optical interconnect is disclosed in U.S. Pat. No. 4,838,630 by utilizing a planar volume Bragg hologram in two dimensions. This technology has no defined signal lines but rather, employs a plane volume element.
Thus, while it is apparent that within the literature there have been a variety of proposals for the use of optical data transfer in the context of computer systems, a need still exists to provide optical bus communication between computer modules.